An interleaver used in an optical communication system is designed to receive signal light of multiple wavelengths (λ1, λ2, . . . , λ2n−1, λ2n, . . . ) and demultiplex the light into signal light in a first wavelength range Λ1 (λ1, λ3, . . . , λ2n−1, . . . ) and signal light in a second wavelength range Λ2 (λ2, λ4, . . . , λ2n, . . . ), and is provided for a repeater or the like in the optical communication system. Such interleavers based on various schemes have been proposed. For example, known interleavers include one that includes a polarizing/separating element, wavelength filter, and polarizing/combining element and one that includes an optical system forming a Mach-Zehnder interferometer. An interleaver having an optical filter including an optical system forming a Michelson interferometer is also conceivable.